Shaping filter for high data rate signalling

Abstract

A shaping filter for a PAM mapper is provided which manipulates incoming PAM code level signals to produce outgoing PAM code level signals whose frequency spectrum is shaped to meet desired criteria. In manipulating input code level values in(t), the shaping filter preferably utilizes previous output signal values without using previous incoming signal values. In a preferred embodiment, the output code level values out(t) are constrained to an output alphabet which is a predefined subset of code levels within the mu -law constellation, and are obtained by manipulating the input signals according to an integer-valued shifting function such that out(t) are code level values within the output alphabet. The preferred shifting function S "shifts" in(t) by a number of places in the output alphabet in the direction opposite the sign of <IMAGE> to provide out(t), where out(t) are mu -law code output level values at times t, and ai are weighting factors chosen to shape the frequency spectrum of out(t). The shaping filter may be used with multidimensional PAM constellations.

I claim: 
     
       1. A PAM mapper, comprising: a) means for receiving incoming information;   b) means for generating an indication of a PAM code from said incoming information, said PAM code being constrained by an input alphabet and having a first frequency spectrum; and   c) filter means for filtering said indication into an indication of a PAM output code having a second frequency spectrum different than said first frequency spectrum, said PAM output code being constrained to discrete values of an output alphabet.   
     
     
       2. A PAM mapper according to claim 1, wherein: said input alphabet is a proper subset of said output alphabet.   
     
     
       3. A PAM mapper according to claim 1, wherein: said input alphabet and said output alphabet are proper subsets of one of μ-law and A-law code levels.   
     
     
       4. A PAM mapper according to claim 3, wherein: said first frequency spectrum is substantially flat at all frequencies, and   said second frequency spectrum is substantially flat at high frequencies, and is attenuated from a first level at said high frequencies by at least 5 dB at 100 Hz, and is attenuated from said first level by at least 10 dB at 50 Hz and therebelow.   
     
     
       5. A PAM mapper according to claim 1, wherein: said filter means determines an integer shift value (k), said integer shift value being used in obtaining said indication of said PAM output code from said indication of said PAM code.   
     
     
       6. A PAM mapper according to claim 5, wherein: said indication of said PAM output code is an octet, and said filter means utilizes said PAM output code indicated by said indication in generating a subsequent integer shift value.   
     
     
       7. A PAM mapper according to claim 6, further comprising: PAM output code to PAM value translation means coupled to said filter, for taking said indication of a PAM output code, and for generating therefrom a PAM value for feedback to said filter means.   
     
     
       8. A PAM mapper according to claim 7, wherein: said shift value k is a function of a summation ##EQU7## where a i  are weighting factors, i is dummy variable of time, N is a positive integer or infinity, and out(t) is said PAM value at time t.   
     
     
       9. A PAM mapper according to claim 8, wherein: said summation is implemented as a simple geometric series such that v(t)=g out(t-1)+g 2  out(t-2)+g 3  out(t-3)+ . . . , with N effectively equalling infinity.   
     
     
       10. A PAM mapper according to claim 9, wherein: said filter means includes memory means for storing a present value for said v(t), and said filter means includes means for updating v(t) according to v(t+1)=g(v(t)+out(t)), where g is a constant.   
     
     
       11. A PAM mapper according to claim 10, wherein: in determining k, said filter means calculates a value for a variable x which is the smaller of one and an absolute value of V/denom, where denom is an expansion factor for said filter means.   
     
     
       12. A PAM mapper according to claim 11, wherein: in determining k, said filter means calculates a value for a variable p which is a function of x.   
     
     
       13. A PAM mapper according to claim 12, wherein: p=c 1  x-c 2  x 2 , where c 1  and c 2  are constants.   
     
     
       14. A PAM mapper according to claim 13, wherein: k is chosen as the minimum of maxshift and an integer rounded value of p, where maxshift is a constant.   
     
     
       15. A PAM mapper according to claim 1, wherein: said incoming information comprises a stream of bits, and said means for receiving groups a plurality of said stream of bits to provide an input label which comprises said indication of a PAM code. 
     
     
       16. A PAM mapper according to claim 1, wherein: said incoming information comprises a μ-law octet, and said means for receiving translates said μ-law octet into an input label which comprises said indication of a PAM code.   
     
     
       17. A PAM mapper, comprising: a) means for generating input PAM code labels constrained by an input alphabet and having a first frequency spectrum;   b) filter means for filtering said input PAM code labels and generating indications of PAM output codes having a second frequency spectrum different than said first frequency spectrum, said PAM output codes being constrained to discrete values of an output alphabet, wherein said input alphabet is a subset of said output alphabet.     
     
     
       18. A method for mapping indications of telecommunications signals, comprising: a) obtaining information indicative of PAM code information, said PAM code information constrained by an input alphabet and having a first frequency spectrum;   b) filtering said information to obtain indications PAM code outputs having a second frequency spectrum different than said first frequency spectrum, said PAM code outputs being constrained to discrete values of an output alphabet,   said input alphabet being a subset of said output alphabet.   
     
     
       19. A method according to claim 18, wherein: said first frequency spectrum is substantially flat at all frequencies, and   said second frequency spectrum is substantially flat at high frequencies, and is attenuated from a first level at said high frequencies by at least 5 dB at 100 Hz, and is attenuated from said first level by at least 10 dB at 50 Hz and therebelow.   
     
     
       20. A PAM mapper according to claim 18, wherein: said filtering determines an integer shift value (k), said integer shift value being used in obtaining said indications of PAM code outputs.